Abstract: A process for preparing optically active indoline-2-carboxylic acid by an optical resolution, which comprises subjecting a racemic ester of (R,S)-indoline-2-carboxylic acid having the general formula [(R,S)-I] to the action of an enzyme or a microorganism having a stereo-selective esterase activity, which is capable of asymmetrically hydrolyzing the racemic ester [(R,S)-I] to give optically active indoline-2-carboxylic acid having the formula [II*] so as to produce the hydrolysis product, i.e. optically active indoline-2-carboxylic acid [II*] and an unreacted optically active ester of indoline-2-carboxylic acid having the general formula [I*], isolating each optically active form, and further, if necessary, hydrolyzing the obtained optically active ester [I*] to give an optical antipode of the acid [II*].According to the process of the present invention, optically active indoline-2-carboxylic acid with a high optical purity can be prepared in a simple process with a good yield.